TOEFL統合ライティング

📖 読解文章

The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, recent evidence suggests that volcanic activity, specifically the Deccan Traps in India, may have been the primary cause. Three main arguments support this volcanic hypothesis. First, the Deccan Traps eruptions lasted for over a million years, releasing enormous amounts of sulfur dioxide and carbon dioxide into the atmosphere. These gases would have caused severe climate change, including global cooling from sulfate aerosols and subsequent warming from CO2, creating a stressful environment for dinosaurs. Second, the timing of the eruptions coincides with the mass extinction event. Radioisotopic dating of lava flows shows that the most intense phase of volcanic activity occurred just before the Cretaceous-Paleogene boundary, where the fossil record indicates a sudden disappearance of non-avian dinosaurs. Third, the fossil record itself reveals a gradual decline in dinosaur diversity in the millions of years leading up to the extinction, suggesting that long-term environmental stress, not a sudden impact, was the driving factor. This gradual decline is inconsistent with the catastrophic, instantaneous effects of an asteroid strike.

🎧 リスニング講義

The reading claims that volcanic activity caused dinosaur extinction, but this view is problematic. First, while the Deccan Traps released gases, their effects were likely not severe enough to cause global mass extinction. Climate modeling shows that the cooling and warming pulses from volcanic emissions were moderate and occurred over long timescales, allowing many species to adapt. In contrast, an asteroid impact would have caused a sudden 'impact winter' with a dramatic drop in temperature, blocking sunlight for months—a scenario far more lethal. Second, the timing argument is flawed. Although volcanism was active near the boundary, the most intense phase actually ended about 200,000 years before the extinction. The final extinction event aligns precisely with the Chicxulub impact, which occurred at the boundary itself. Third, the idea of a gradual decline is based on incomplete fossil sampling. When corrected for sampling biases, dinosaur diversity remained high until the very end. The apparent decline is an artifact of poor preservation in certain rock layers. Thus, the asteroid impact remains the most plausible cause, as it explains the sudden, catastrophic nature of the extinction.